A common concern when growing algae is protection of the algae from biological contaminants. Within the algae growing industry this is known as crop protection. The current state of scaled algae production, typical for biofuels, for example, is to first start a small pure culture (axenic or uni-algal) on an agar plate or slant in sterile laboratory conditions. Large-scale algae production, as shown in FIG. 1, will commonly progressively scale up batch (i.e., non-continuous) cultures of algae from culture tubes or flasks (<100 ml), to columns (˜100 ml-800 ml) 100, to panels (15 L to 1,500 L) 102 or outdoors, and finally as shown as in FIG. 2 to outdoor ponds (1,000-150,000+L; and/or photo-bioreactors) 200. Due to the continuous threat of biological contamination, algae production has mostly been limited to batch cultivation, as opposed to the more productive continuous cultivation. A hurdle to continuous algae cultivation is prevention or minimization of biological contamination.
Each stage of the scale-up sequence provides the inoculum (i.e., the substance used for inoculation) for the subsequent stage, and care must be taken to identify biological contamination and confirm the health of the culture prior to transfer and continued scale-up. Signs of contamination include, among others, discoloration, unusual odor, biofouling, foam production, or auto-flocculation. Causes of biological contamination include, among others, the presence of predator protists (amoeboids, ciliates and flagellates), non-beneficial bacteria, fungi, filamentous cyanobacteria, or undesirable species of algae.
After inspection (commonly at the microscopic level) to confirm that the culture is sufficiently free of biological contaminants at the end of the batch culture period (which can take anywhere from a few days to a few weeks depending on growth conditions), the contents of a column or panel is then transferred to a panel or pond (respectively) that only contains growth media, and the process continues. Carbon dioxide and nutrients (e.g., nitrogen, phosphorus, and/or other micronutrients) in excess allow for uninhibited logarithmic growth (log-growth) until the concentration of algae becomes self-limiting due to shading. With the final cultivation step for algal growth for biofuel production, the crop is starved of nitrogen, stimulating the storage response that results in hyper-accumulation of carbon as an intracellular lipid, at which point the crop is harvested. Harvesting involves dewatering the entire contents of the pond or photo-bioreactor and passing through a combination of centrifuges, settlers, and/or dissolved air floatation technologies to concentrate and collect the algal biomass.
After inoculation of any step, the sheer mass of algae is relied on to out-compete any biological contaminants during log-growth phase, as long as substantially the entire volume is transferred to the next stage or harvested expeditiously. Still, both the batch inoculum stages and final pond or photo-bioreactor stage are subject to biological contamination. If there are enough of these unwanted organisms, and/or they grow fast enough (Rotifers can consume 70,000 algal cells per day, for example), the inoculum batch must be discarded or the pond or photo-bioreactor must be harvested prematurely before the entire crop is completely infected and useless. A particular hurdle to algae cultivation, including continuous cultivation, is therefore prevention or minimization of predation.